1. Field of the Invention
This invention relates to a transformer suitable for use in a power supply for a three-phase output circulating current type cycloconverter having a twelve-pulse bridge arrangement.
2. Description of the Prior Art
FIGS. 10, 11A, 11B and 11C, and 12 illustrate a basic arrangement of a conventional single-phase output circulating current type cycloconverter 1. Referring to FIG. 10, an electrical arrangement of the cycloconverter 1 is shown. The cycloconverter 1 comprises a three-phase transformer 3 connected to a three-phase alternating current (AC) power supply 2 and a converter section 8 including a positive group converter 4 and a negative group converter 5 both of which are connected via circulating current limiting reactors 6 and 7 in reverse parallel to each other. The positive group converter 4 comprises six thyristors 9 to 14 connected into a three-phase bridge configuration, whereas the negative group converter 5 comprises six thyristors 15 to 20 connected into a three-phase bridge configuration. The transformer 3 includes a primary winding 21 connected in a three-phase configuration and two secondary windings 22 and 23 each of which is connected in a delta configuration, for example. One secondary winding 22 is connected to input terminals of the positive group converter 4, whereas the other secondary winding 23 is connected to input terminals of the negative group converter 5. A load 24 is connected between neutral points of the reactors 6 and 7.
In the above-described cycloconverter 1, gate signals having predetermined patterns are supplied to the thyristors 9 to 14 of the positive group converter 4 and the thyristors 15 to 20 of the negative group converter 5, respectively. As a result, substantially sinusoidal voltages e.sub.op and eon as shown by bold solid lines in FIGS. 11A and 11B are generated between output terminals Tp1 and Tp2 of the positive group converter 4 and between output terminals Tn1 and Tn2 of the negative group converter 5, respectively. A substantially sinusoidal voltage e.sub.o, which is equal to a mean value of the voltages e.sub.op and e.sub.on as shown by bold solid line in FIG. 11C, is obtained between both terminals of the load 24. Each of thin solid lines in FIGS. 11A to 11C shows a voltage of the three-phase AC power supply 2. Broken lines in FIGS. 11A to 11C show fundamental wave components of the voltages e.sub.op, e.sub.on and e.sub.o respectively.
The input voltage is thus supplied into the cycloconverter 1 from the three-phase AC power supply 2 when the gate signals are supplied to the thyristors 9 to 20 respectively. A power supply frequency of the input voltage is directly converted to a lower frequency in a predetermined range such that a single-phase AC voltage is delivered. Accordingly, the cycloconverter 1 serves as a frequency converting circuit.
FIG. 12 shows another conventional cycloconverter 25 including a converter section 26. The converter section 26 comprises a positive group converter including a first positive group converter 27a and a second positive group converter 27b both of which are connected to each other so as to form a cascade. The converter section 26 further comprises a negative group converter including a first negative group converter 28a and a second negative group converter 28b both of which are connected to each other so as to form a cascade. Each of the positive group converters 27a and 27b has the same arrangement as the above-described positive group converter 4, and each of the negative group converters 28a and 28b has the same arrangement as the above-described negative group converter 5.
A three-phase transformer 29 includes primary windings 30a and 30b, a first positive group winding 31a and a first negative group winding 32a both of which serve as secondary windings corresponding to the primary winding 30a as shown in FIG. 14. The transformer 29 further includes a second positive group winding 31b and a second negative group winding 32b both of which serves as secondary windings corresponding to the primary winding 30b, as shown in FIG. 14. The first positive and negative group windings 31a and 32a are connected to the first positive and negative group converters 27a and 28a respectively. The second positive and negative group windings 31b and 32b are connected to the second positive and negative group converters 27b and 28b respectively.
For example, each of the first positive and negative group windings 31a and 32a is connected in a delta configuration, and each of the second positive and negative group windings 31b and 32b is connected in a wye configuration. This arrangement results in a phase difference of 30 degrees between the first and second converters of the positive and negative groups respectively. Accordingly, the cycloconverter 25 reduces harmonic components of the output voltage e.sub.o more than the cycloconverter 1. The converter section 8 of the cycloconverter 1 has a six-pulse bridge arrangement, whereas the converter section 26 of the cycloconverter 25 has a twelve-pulse bridge arrangement. The above-described cycloconverter 25 is connected in a three-phase configuration such that a three-phase output cycloconverter 33 having the twelve-pulse bridge arrangement as shown in FIG. 13 is composed.
Various transformer arrangements have conventionally been used for the above-described cycloconverter 33 in the prior art. FIG. 13 shows one of the prior-art transformer arrangements. The above-described three transformers 29 are provided in the respective phase converter sections 26. FIG. 14 shows a winding arrangement for one of legs of an iron core of each transformer 29. More specifically, on an upper portion of one leg 34p of a three-legged core 34 are wound an innermost first positive group winding 31a, a primary winding 30a and an outermost first negative group winding 32a in this order as viewed in FIG. 14. Further, on a lower portion of the leg 34p are wound an innermost second positive group winding 31b, a primary winding 30b and an outermost second negative group winding 32b in this order as viewed in FIG. 14.
The primary windings 30a and 30b are connected in parallel to each other and further connected to the respective primary windings 30a and 30b wound on the other two legs (not shown) each in a three-phase configuration, further connected to the three-phase AC power supply 2. Furthermore, the first positive and negative group windings 31a and 32a are connected to the respective first positive and negative group windings 31a and 32a of the other two legs each in a delta configuration. The second positive and negative group windings 31b and 32b are connected to the respective second positive and negative group windings 31b and 32b of the other two legs each in a wye configuration.
FIG. 15 shows an electrical arrangement of another prior-art cycloconverter 35. The cycloconverter 35 is constructed so that two three-phase transformers 36a and 36b apply predetermined AC voltages to the respective phase converter sections 26. FIG. 16 shows a winding arrangement for one of legs of an iron core of the transformer 36a. More specifically, on an upper portion of one leg 37p of a three-legged core 34 are wound an innermost first positive group winding 31a, a primary winding 30a and an outermost first negative group winding 32a in this order as viewed in FIG. 16. Further, on a middle portion of the leg 37p are wound an innermost first positive group winding 31a', a primary winding 30a' and an outermost first negative group winding 32a' in this order as viewed in FIG. 16. Additionally, on a lower portion of the leg 37p are wound an innermost first positive group winding 31a", a primary winding 30a" and an outermost first negative group winding 32a" in this order as viewed in FIG. 16.
The primary windings 30a, 30a' and 30a" are connected in parallel to one another and further to primary windings 30a, 30a' and 30a" of the other two legs (not shown) each in a three-phase configuration. The secondary windings 31a, 31a' and 31a " are connected to respective secondary windings 31a, 31a' and 31a" of the other two legs each in a delta configuration and further to first positive group converters 27a of the respective phases. The secondary windings 32a, 32a' and 32a" are connected into a delta configuration in the same manner as described above and further to first negative group converters 28a of the respective phases. The transformer 36b has the same arrangement as described above except that the secondary windings 31b, 31b', 31b", 32b, 32b' and 32b" are connected in a wye configuration.
FIG. 17 shows an electrical arrangement of further another prior-art cycloconverter 38. The cycloconverter 38 is constructed so that a single three-phase transformer 39 applies a predetermined AC voltage to each phase converter section 26. FIG. 18 shows a winding arrangement for one of legs of an iron core of the transformer 39. More specifically, on an upper portion of one leg 40p of a three-legged core 40 are wound the same windings as those wound on the upper portion of the leg 37 of the above-described transformer 36a (see FIG. 16). Further, on a lower portion of the leg 40p are wound the same windings as those wound on the lower portion of the leg of the above-described transformer 36b.
In each of the aforesaid transformers 29, 36a, 36b and 39, each primary winding is interposed between the positive and negative group windings such that these windings are magnetically coupled close with one another. Accordingly, a load current flows into the primary windings during energization to either positive or negative group windings as disclosed in Japanese Patent Application Publication No. 63-186564A published on Aug. 2, 1988. Consequently, since a ratio of use of the primary windings to the secondary windings is improved, a total capacity of the primary windings can be rendered 1/2 times smaller than a total capacity of the secondary windings. Further, the three-legged cores 34 and 40 are excited by a twelve-pulse current through an overall period in the respective transformers 29 and 39. Consequently, harmonics can be reduced as compared with a case where the core is excited by a six-pulse current and accordingly, a core loss can also be reduced.
Consider a case where sets of the positive group windings, primary windings and negative group windings wound on the legs 34p, 37p and 40p of the transformers 29, 36a (36b) and 39 of the respective conventional cycloconverters 33, 35 and 38 have the same dimensions. In this case, an amount of core material used is rendered smaller as the number of transformers is decreased, and with this, no-load loss is reduced. The arrangement of the cycloconverter 38 as shown in FIG. 17 is superior in this respect.
Further, all the secondary windings 31a-32b" of the transformer 39 of the cycloconverter 38 are wound on the single three-legged core 40 so as to form the same magnetic circuit with the core. Accordingly, the core 40 is excited by the twelve-pulse current through the overall period including a period in which the positive group converters 27a and 27b supply positive half-cycle voltages and a period in which the negative group converters 28a and 28b supply negative half-cycle voltages. As a result, the transformer 39 has an advantage that the core loss is reduced. This also applies to each of the transformers 29 of the cycloconverter 33 having the first and second positive group windings 31a and 31b and the first and second negative group windings 32a and 32b.
However, the above-described transformer 39 has twelve secondary windings per leg. With respect to the middle leg 40q, a space utilized to extend lead wires is limited to two opposite directions as shown in FIG. 19 which is a schematic plan view of the transformer 39. As a result, it is difficult to extend the twelve lead wires regarding the middle leg 40q. Accordingly, the arrangement of the transformer 39 has not been employed hitherto.
On the other hand, the transformer 36a of the cycloconverter 35 as shown in FIG. 15 has the secondary windings 31a to 32a" connected to the first positive and negative group converters 27a and 28a of the respective phases. Further, three secondary windings 31a of the respective phases are connected in the delta configuration. All the other secondary windings 32a to 32a" of the respective phases are also connected each in the delta configuration. Accordingly, the transformer 36a is excited by the six-pulse current through the overall period and accordingly has a disadvantage that the core loss is increased. This also applies to the transformer 36b.
In view of the above-described disadvantage, the prior art has proposed a cycloconverter 41 having a modified arrangement of the secondary windings of the transformers 36a and 36b as shown in FIG. 20. A transformer 42a of the cycloconverter 41 includes parallel connected primary windings 30a, 30a' and 30" of the respective phases, first positive group windings 31a, 31a' and 31" of the respective phases and first negative group windings 43a, 43a' and 43a" of the respective phases. The first positive group windings 31a of the respective phases are connected in a delta configuration. The other first positive group windings 31a' and 31a" of the respective phases are each connected in a delta configuration, too. The first negative group windings 43a of the respective phases are connected in a wye configuration. The other first negative group windings 43a' and 43a" of the respective phases are each connected in a wye configuration, too. Further, a transformer 42b also includes second negative group windings 43b, 43b' and 43b" of the respective phases which are each connected in the delta configuration instead of the wye configuration. According to the above-described arrangement, a circulating current flowing into the cycloconverter 41 is a twelve-pulse current. However, since this circulating current component is small, each of the transformers 42a and 42b is still excited by the six-pulse current and the core loss cannot be reduced much.
To overcome the above-described drawback, the prior art has further proposed a cycloconverter 44 having a further modified arrangement of the secondary windings of the transformers 42a and 27b as shown in FIG. 21. The transformer 45a includes secondary windings connected to the positive group converters 27a and 27b of the respective phases. More specifically, the transformer 45a includes parallel connected primary windings 30a, 30a' and 30a" of the respective phases, first positive group windings 31a, 31a' and 31a" of the respective phases which are each connected in a delta configuration, and second positive group windings 31b, 31b' and 31b" of the respective phases which are each connected in a wye configuration. Further, a transformer 48b also includes first negative group windings 43a, 43a' and 43a" of the respective phases which are each connected in the wye configuration and second negative group windings 43b, 43b' and 43b" of the respective phases which are each connected in the delta configuration. Consequently, each of the transformers 45a and 45b is excited by a twelve-pulse current through the overall period. However, a required total capacity of the primary windings of each transformer is equal to a total capacity of the secondary windings. This renders the size of each transformer larger than those of the above-described transformers 36a, 36b, 42a and 42b and accordingly increases the manufacturing cost of the cycloconverter.